Technical Field
The present disclosure relates to an antenna device having a patch antenna.
Background Art
A patch antenna formed on a dielectric substrate has been used for a radar apparatus, for example, on a mobile unit, including a vehicle and an airplane, for monitoring the surroundings of the mobile unit. Commonly, in the configuration of a patch antenna, a patch radiating element (a patch-shaped conductor) is formed on a dielectric substrate. Generally, a conductor part serving as a ground plane is formed on a face of the dielectric substrate (in the following, referred to as “a substrate rear face”) on the opposite side of a face on which the patch radiating element is formed (in the following, referred to as “a substrate front face”). Also on the substrate front face, a conductor part is sometimes widely formed to substrate end portions in addition to the patch radiating element.
In a patch antenna in this configuration, upon operating the patch antenna, electric current (surface current) flows through the surface of the ground plane due to an electric field formed across the patch radiating element and the ground plane. The surface current is propagated to the substrate end portions, and diffracted at the substrate end portions. Because of the influence of the diffracted waves, radiation (emission) occurs from the substrate end portions. In the case where a conductor part is formed on the substrate front face, the surface current also flows through the conductor part to cause radiation from the substrate end portions. Radiation from the substrate end portions due to this surface current is unnecessary radiation that adversely affects the performance of the patch antenna. In other words, radiation from the end portions disturbs the directivity of the patch antenna.
JP-T-2002-510886 discloses a technique to reduce surface current flowing through a ground plane. Specifically, a plurality of conductive patches is formed around a patch radiating element on the substrate front face of a dielectric substrate. The conductive patches are each electrically connected to a ground plane on the rear face of the substrate with a columnar connector (in the following, referred to as “a conducting via”). The structure configured of the conductive patch and the conducting via has a band gap (an electromagnetic band gap) that prevents the propagation of the surface current from flowing through the ground plane at a specified frequency. In the following, the structure configured of the conductive patch and the conducting via is referred to as “an EBG”.
In this manner, forming a large number of EBGs around the patch radiating element allows a reduction in the propagation of the surface current to the substrate end portions. Thus, the disturbance in the directivity of the patch antenna can be reduced.